SI Joint Pain: Diagnosis, Injection & Fusion

SI Joint Diagnostic and Therapeutic Injection Technique

Intra-Articular SI Joint Injection

1. Patient positioning: Prone on fluoroscopy table
2. Fluoroscopic guidance: Use AP or oblique views to visualize the SI joint — the inferior aspect of the joint is the typical entry point
3. Needle selection: 22-25 gauge, 3.5-inch spinal needle
4. Target: Inferior aspect of the SI joint under fluoroscopy
5. Joint entry: Advance the needle into the joint, confirming position with contrast arthrogram
6. Arthrogram: Inject contrast to confirm intra-articular position — should see dye filling the joint space
7. Diagnostic injection: Small volume (1-2 mL) of local anesthetic to confirm SI joint as pain source
8. Therapeutic injection: Add corticosteroid (40-80 mg triamcinolone or 6-8 mg dexamethasone)

Critical Point: Image Guidance Is Required

Blind SI joint injections have extremely poor accuracy — only 12-22% intra-articular placement rate without imaging guidance. Fluoroscopic (or CT/ultrasound) guidance is mandatory.

IPSIS Fluoroscopic Technique Details

The IPSIS Technical Manual provides detailed guidance on achieving reliable SI joint access:

Fluoroscopic Setup and Target Acquisition:

• Apply 10-25 degrees of cephalad tilt so the inferior joint projects just above the superior pubic ramus; this separates posterior from anterior joint planes vertically
• Obtain an adjusted AP view (typically 5-20 degrees contralateral oblique) to superimpose the anterior and posterior joint margins at the caudal third
• Then rotate ipsilaterally to separate the joint lines horizontally — the medial silhouette usually represents the posterior (accessible) joint line
• Adjust obliquity until the inferior one-third of the medial joint line cortex is maximally crisp, indicating the beam is parallel to the posterior opening

Needle Placement Strategy:

• Target 1-2 cm cephalad to the inferior aspect of the joint
• Consider a slight medial-to-lateral trajectory given the typical joint orientation and unseen osteophytes
• Advance to sacral periosteum first (just medial to the joint line to prevent overpenetration), then redirect laterally into the joint
• Joint entry is recognized by loss of bony resistance followed by a subtle additional loss of resistance — the feeling of articular glide between opposing surfaces

Arthrogram Interpretation:

• The joint capacity is typically 2.0 mL or less — inject only 0.1-0.2 mL of contrast initially
• Initial intra-articular flow will cap the inferior aspect of the lucent joint space and fill a slightly expanded inferior recess
• True intra-articular spread appears as a thin line (<1 mm) centered between the cortical joint margins
• Normal sacculations may extend peripherally from the joint space
• If resistance to injection is encountered, rotate or slightly withdraw the needle (bevel may be in articular cartilage)
• Total injectate for diagnostic blocks should not exceed 1.5 mL; total volume (including contrast) should not exceed 2 mL

Osteophyte Challenge:

• Osteophytes typically arise from the iliac side, arcing lateral to medial over the inferior joint
• These are not visible on fluoroscopy because they are perpendicular to the X-ray beam, but they are readily apparent on CT
• A default medial-to-lateral needle trajectory may allow passage underneath unseen osteophytes
• Pre-procedure CT review is highly valuable for identifying these obstacles

Capsular Defects and Extra-Articular Spread:

• Capsular defects are present in 20-30% of patients; injectate may flow through ventral capsular foramina to the lumbosacral trunk or S1 foramen
• This extra-articular anesthetic spread can cause temporary leg numbness/weakness and compromises block specificity

SI Joint Lateral Branch Blocks

This is a newer diagnostic/prognostic tool used before SI joint RFA:

1. Target nerves: Lateral branches of S1-S3 dorsal rami (and sometimes L5 dorsal ramus)
2. Technique: Small-volume local anesthetic blocks at each lateral branch target
3. Fluoroscopic targets: Along the lateral ala of the sacrum near the posterior sacral foramina
4. Purpose: Diagnose posterior SI joint complex pain and predict response to RFA

IPSIS Sacral Lateral Branch Block (SLBB) Technique

The IPSIS manual distinguishes SLBBs from intra-articular SI joint blocks — they target different structures and answer different clinical questions:

Anatomy of the Posterior Sacral Network (PSN):

• The posterior SI joint complex (ligaments, posterior capsule, syndesmosis) is innervated by the PSN — a plexus formed by lateral branches of S1-3 plus variable contributions from L5 dorsal ramus and S4
• The PSN innervates the short and long posterior SI ligaments, interosseous SI ligaments, and part of the sacrotuberous ligament
• The middle cluneal nerves also arise from the PSN, providing cutaneous sensation to the medial buttock

Critical Fluoroscopic Landmark — The Arcuate Line:

• The perceived lateral margin of the sacral foramen on fluoroscopy is actually deep within the foramen (the true lateral margin is oblique to the X-ray beam and invisible)
• The inflection point of the arcuate line approximates the true anatomic center of the dorsal foramen
• All SLBB targets must be placed 8-10 mm lateral to this inflection point to avoid unintended foraminal needle placement

Clockface Target Technique (multi-site, multi-depth):

• Uses clockface coordinates around each dorsal sacral foramen, with targets 8-10 mm lateral to the arcuate line inflection:
  • S1: 6 and 7:30 (left) / 4:30 and 6 (right)
  • S2: 6:30, 8, and 9:30 (left) / 2:30, 4, and 5:30 (right)
  • S3: 9:30 and 11 (left) / 1 and 2:30 (right)
• At each target: inject 0.2 mL anesthetic on periosteum, withdraw one bevel-length, inject 0.2 mL more
• Validated multi-site, multi-depth approach produces reliable anesthesia of the dorsal SI joint complex (single-site, single-depth blocks were shown to be inadequate)

Palisade Technique (alternative):

• Six or more needles placed no more than 1 cm apart along a straight line 8-10 mm lateral to the arcuate line
• Spans from just above the S1 foramen to the mid-lower margin of S3 (or S4)
• Creates a continuous strip of anesthesia covering all pertinent lateral branches

Confirming a Successful Block:

• A technically adequate SLBB should produce hypoesthesia (numbness) of the medial buttock skin in the middle cluneal nerve distribution
• This is a useful bedside check to confirm the block reached the intended targets
• The L5 dorsal ramus must also be blocked to ensure complete posterior SI joint complex anesthesia

SI Joint RFA Technique

• Technology: Cooled RF is preferred over conventional RF for SI joint ablation
• Multiple electrodes: Required to cover the variable course of the lateral branches
• Targets: S1-S3 lateral branches along the sacrum
• Limitation: RFA only addresses posterior innervation — the anterior SI joint (innervated by L2-S2 ventral rami) remains innervated

IPSIS SLBRFN (Sacral Lateral Branch Radiofrequency Neurotomy) Details

The IPSIS manual describes two primary RF approaches for SI joint denervation:

Clockface Technique (large-lesion monopolar/cooled RF):

• Uses the same clockface targets as the SLBB technique (8-10 mm lateral to arcuate line inflection)
• Large-gauge cannulae (16-18G) with 10 mm active tips, or 17G internally cooled RF probes
• Cooled RF generator set to 60 degrees C, but intralesional temperature reaches 80 degrees C for 150 seconds
• Conventional monopolar lesions at 80-85 degrees C for 180 seconds at each target
• All clockface targets at each foramen must be lesioned for proper denervation

Palisade Technique (bipolar RF strip lesioning):

• Six or more parallel large-gauge cannulae placed no more than 1 cm apart
• Bipolar lesioning between adjacent cannulae creates a continuous coagulation strip
• Avoids the bow-tie effect (diminished lesion in the center between electrodes)
• 80-85 degrees C for 180 seconds; some practitioners use higher temperatures (up to 90 degrees C) or longer times

Safety Note — Foraminal Cannula Placement:

• The same fluoroscopic illusion that affects SLBBs applies to RFN: the perceived lateral foraminal margin is deep within the foramen
• RF lesioning within the foramen can injure the ventral ramus
• Cannulae that appear to project deep on lateral imaging may actually be correctly placed on the curved dorsal sacral surface

L5 Dorsal Ramus RFN:

• Should be performed concurrently for complete posterior SI joint complex denervation

Percutaneous SI Joint Fusion

• iFuse (SI-BONE): Triangular titanium implants placed percutaneously across the SI joint under fluoroscopic guidance
• Approach: Lateral approach through the ilium into the sacrum
• Competitors: Multiple companies now offer various designs — screws, implants, allograft options
• Some newer devices come in from posterior, and companies differentiate themselves with terms like "stabilize" or "distract" rather than "fuse"